1. Field of the Invention.
This invention relates to electronic circuits for generating complex time-varying analog signal waveforms and more particularly to an electronic circuit which employs digital to analog signal conversion techniques to generate complex time-varying analog signal patterns such as electrocardiogram signals and blood pressure transducer signals.
2. Prior Art.
Many different types of electronic circuits for generating time-varying analog signals are in common use. Circuits for generating sinusoidal waveforms are well known in the art, but the generation of periodic complex waveform is comparatively difficult. One method of generating a periodic complex waveform is by generating a series of harmonically related sinusoidal waveforms and summing the series to thereby produce the complex waveform. Fourier analysis of the particular complex waveform is used to determine the particular frequency, phase and amplitude of each sinusoidal waveform. Since this method requires a plurality of separate signal generating circuits, each producing a waveform having a certain frequency, phase and amplitude, the method is complicated and expensive.
Another method of generating periodic complex waveforms is by using a plurality of circuits, with a separate circuit to generate each specific portion of the waveform. The outputs of the separate circuits are summed to thereby produce the entire complex waveform. However, in order to insure that the periodic relationship between each specific portion of the waveform is correct, it is necessary to detect the completion of the generation of each portion of the waveform before the generation of the next portion. This detection is typically accomplished by sensing the point in time when the amplitude of the preceeding portion of the waveform has crossed a point such as the zero level. The detection of this crossing thereby triggers the succeeding circuit, and this cycle is repeated for each portion of the waveform until the completion of the complex waveform. This method, however, is sensitive to offset levels in the circuitry and requires complex circuitry.